Conservation, Research and Education Opportunities International

Protecting whales in the Pacific Northwest by assessing and reducing “cryptic mortality”

Oceans Initiative

Many US laws and policies use “best available science” to determine when a whale or dolphin population requires protection from ship strikes or entanglement in fishing gear. These protection measures may only be triggered when the science shows that we are exceeding some allowable harm threshold, but carcasses detected on the beach reflect only the tip of the iceberg. Many forms of mortality (e.g., ship strikes or entanglement in crab gear) are inherently cryptic in nature. Following Deepwater Horizon, researchers used modeling to determine that for every dolphin or whale carcass found on the beach, an additional 50-250 disappeared at sea. We propose to apply those methods to estimate how much we could be underestimating mortality from ship strikes and entanglement in crab gear – i.e., cryptic sources of mortality that do not lend themselves to traditional fisheries observer programs. Working with Cascadia Research Collective, we will estimate carcass detection rates in gray, humpback, blue and fin whale populations on the US West Coast.

We will generate correction factors to scale up minimum counts of observed human-caused mortality in the four whale species to assess whether plausible estimates of total human-caused mortality (i.e., a more accurate estimate of the true death toll) are large enough to warrant mitigation under the US Marine Mammal Protection Act or state-managed fisheries (e.g., Dungeness crab). We propose two ways to estimate the number of whale deaths that go undetected. With only one abundance estimate, we will use life-history tables to estimate how many animals should be dying in a given year. Then, we will compare that to how many stranded carcasses detected. That proportion is a carcass detection rate. For data-rich species, for which we have time-series of abundance estimates, we will model population growth rate with a parameter for cryptic mortality that can vary over time (e.g., as countries invest in marine mammal stranding responses, or as factors related to body condition or cause of death). This approach models how low carcass detection rate could be, given observed population trends.

Our use-inspired science is designed strategically to deliver knowledge to the NMFS and WDFW staff with statutory authority to use “best available science” to mitigate harm from shipping or fisheries. NMFS and WDFW will use our estimates of carcass detection rate to explicitly account for cryptic mortality in their annual assessment reports for the four whale species. By accounting for total mortality, including cryptic mortality, we will detect anthropogenic problems sooner, and form Take Reduction Teams earlier, than we would otherwise. This leads to more precautionary management, and leverages the power of NMFS and WDFW policies to reduce harm before it becomes irreversible. For baleen whales, we know that entanglement is a problem but we are underestimating it. Our correction factors will lead to earlier intervention. The method is broadly transferable, so we can advance conservation of these whales, and set a precedent for NMFS to start using this in management as a matter of course. ($15,000)